1. Field of the Invention
This invention relates to coating substrates, especially preformed plastic substrates, and barrier coating of plastic containers. For instance, polyethylene terephthalate bottles are coated with a copolymer of polyvinylidene chloride to provide the bottles with a gas barrier coating. More particularly, conventional airless spray equipment is employed to provide the surface of polyethylene terephthalate containers with a high quality, uniformly transparent barrier coating to substantially reduce or prevent the passage of gases through the walls of the containers.
2. Description of the Art
Plastic containers for beverages made of polyethylene terephthalate (commonly referred to as PET bottles or containers) have become popular for a number of reasons including their light weight; their strength and capacity to hold beverages, including carbonated beverages such as soft drinks and colas; their lack of toxicity and the economies of materials and methods by which the containers can be manufactured. Typically, these containers are made by a process called "blow molding" in which a preform or parison is heated and stretched both axially and radially by air pressure in a mold to the desired shape of the container. Such biaxially oriented PET containers are strong and have good resistance to creep, i.e., they maintain their dimensions even under the internal pressure caused by gases in the liquid inside the bottles. Moreover, the containers are relatively thin walled, and hence are lightweight but, nevertheless, are capable of withstanding without undue distortion over the desired shelf life of the product the internal pressure exerted by a carbonated liquid, such as soft drinks and colas.
However, a major problem with such thin-walled PET containers are that they are permeable to gases such as carbon dioxide and oxygen. That is, with PET containers, these gases are capable of migrating or passing through the wall of the container due to the pressure differential between the gas inside and the pressure outside of the container. Thus, in the case of bottles containing carbonated liquids the pressurizing carbon dioxide in the liquid which is typically at a pressure on the order of 60-75 pounds per square inch gauge (psig) can migrate through the walls of the container and be released. This migration of carbon dioxide takes place over a period of time. As a result, the carbonated liquid gradually loses its carbon dioxide; and, when the bottle is opened, the beverage lacks carbonation or is what is commonly referred to as being "flat". Conversely, PET containers are permeable to oxygen which permits the oxygen in room air to migrate through the walls and into the container which can cause spoilage of certain comestibles contained in the containers which are subject to deterioration by the presence of oxygen. This then affects the flavor and quality of the container contents.
At present, one commercial manufacturer and bottler of carbonated soft drinks requires that the loss of pressure in PET bottles at room temperature (23.degree. C. 50% r.h.) over a sixteen week period be no more than 15%, e.g., no more than 9 psig starting from 60 psig. This is referred to as the "shelf life" of the bottle, i.e., how long the bottle and its contents can be held prior to sale without unacceptable deterioration of product quality. With uncoated PET bottles, in some cases, the time required to distribute the bottles to the point of sale alone can exceed this shelf life for up to one-half of the United States.
The problem of gas permeability in PET bottles or containers is particularly severe where the container is relatively small; and, as a result, the ratio of the surface area of the container to the volume of the contents is larger than with larger containers. An example of such a container is a 1/2 liter size container, which is a desirable size for carbonated liquids such as soft drinks and colas.
For the foregoing reasons, prior workers in the art have found it desirable to provide PET containers with a layer of material which has a low vapor and gas permeability which thus provides a coating or barrier on the surface of the containers to prevent the passage of gases therethrough. One material which has been employed by prior art workers to provide such a barrier coating is a copolymer of vinylidene chloride (commonly referred to as PVDC). This material is a polymer which may be applied as a latex, i.e., an aqueous polymer dispersion and thereafter dried to form the desired barrier coating. Various techniques have been employed to apply barrier coatings of PVDC latices including the coating of PET preforms prior to blow molding and roll coating of the surface of blow molded PET containers.
Although PVDC has been successfully applied to the surface of PET containers by such methods as roller coating, such a process is not particularly efficient or economical in that it does not lend itself to high speed production rates. That is, in industry, PET bottles are produced at a rate of 700 to 1800 bottles per minute. Thus, an efficient and economic coating process should provide the PET bottle with a PVDC coating at a rate of 300 bottles per minute or greater. Currently, the cost of equipment to satisfy this production rate or even higher rates by roller coating is inordinately high.
Prior patents disclose a number of techniques for coating polymer latices including roller coating, brush coating, dip coating, spray coating, electrostatic coating, centrifugal coating, cast coating, and others. For example, recently issued U.S. Pat. No. 4,370,368 refers to such techniques in general and, in the operating examples, again generally refers to them as suitable ways to deposit the latex on a preformed plastic surface usually with a wetting property-improving preliminary treatment such as anchoring layers or the like. Specific reference is made in this patent to "spray coating" of latex in the examples, but for instance, in Examples 10 and 13, the plastic bottle is first dip-coated to provide an anchoring agent before spray-coating with a PVDC latex. Other patents have dealt with the problems of attempting to spray coat plastic bottles with latices such as U.S. Pat. Nos. 3,696,987; 3,804,663; 4,004,049 and British Pat. No. 2,014,160. There may be other patents of interest as background to this invention, but the above are merely cited not to completely develop the prior art but to help illustrate and highlight this invention. For instance, U.S. Pat. No. 3,804,663 approaches the known problems of latex coating by spinning the coating during spraying thereby causing centrifugal force to distribute and/or hold the dispersion uniformly on the wall and heating to fusion while continuing to spin. U.S. Pat. No. 4,004,049 deals with sprayable latex adhesives with the objective to break the emulsion upon spraying, i.e., atomize and destabilize the latex to produce a pebbly, particulate pattern which requires little or no drying. While the aforementioned remaining patents again generally mention spraying, no attention is apparently given to problems associated with such techniques.
It is known in industry that spray coating is an efficient and high speed method of applying coating materials in a liquid form to substrates. However, as evidenced by the above patents, special considerations apply when attempting to spray polymer latices. It would be highly desirable if a process could be provided for using conventional equipment to coat such latices on plastic bottles such as PET. But, applicants have found that when PET bottles are spray coated with aqueous polymer dispersions of PVDC according to conventional spray coating techniques the resulting coating is very non-uniform and, when dried, the coating is not uniformly transparent such that it distorts the surface appearance of the bottle and thus is totally commercially unacceptable. Moreover, the pressure losses from such spray coating containers are unacceptably high. That is, in today's commercial applications, the PVDC or any polymer barrier coating on PET containers must be highly uniform, smooth, clear, uniformly transparent, glossy, not subject to delamination, and not cracked or crazed as well as substantially impermeable to gas migration. Otherwise, the coated container is simply unusable commercially. Prior to the present invention, a process has not been available to coat with conventional spray equipment and processing PET containers with PVDC which produces barrier coatings meeting these requirements.